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Energy Material Advances / 2021 / Article

Research Article | Open Access

Volume 2021 |Article ID 9761263 | https://doi.org/10.34133/2021/9761263

Derek Hao, Jiawei Ren, Ying Wang, Hamidreza Arandiyan, Magnus Garbrecht, Xiaojuan Bai, Ho Kyong Shon, Wei Wei, Bing-Jie Ni, "A Green Synthesis of Ru Modified g-C3N4 Nanosheets for Enhanced Photocatalytic Ammonia Synthesis", Energy Material Advances, vol. 2021, Article ID 9761263, 12 pages, 2021. https://doi.org/10.34133/2021/9761263

A Green Synthesis of Ru Modified g-C3N4 Nanosheets for Enhanced Photocatalytic Ammonia Synthesis

Received02 May 2021
Accepted13 Aug 2021
Published06 Sep 2021

Abstract

Nitrate is a crucial environmental pollutant, and its risk on ecosystem keeps increasing. Photocatalytic conversion of nitrate to ammonia can simultaneously achieve the commercialization of environmental hazards and recovery of valuable ammonia, which is green and sustainable for the planet. However, due to the thermodynamic and kinetic energy barriers, photocatalytic nitrate reduction usually involves a higher selectivity of the formation of nitrogen that largely limits the ammonia synthesis activity. In this work, we reported a green and facile synthesis of novel metallic ruthenium particle modified graphitic carbon nitride photocatalysts. Compare with bulk graphitic carbon nitride, the optimal sample had 2.93-fold photocatalytic nitrate reduction to ammonia activity (2.627 mg/h/gcat), and the NH3 selectivity increased from 50.77% to 77.9%. According to the experimental and calculated results, the enhanced photocatalytic performance is attributed to the stronger light absorption, nitrate adsorption, and lower energy barrier for the generation of ammonia. This work may provide a facile way to prepare metal modified photocatalysts to achieve highly efficient nitrate reduction to ammonia.

1. Introduction

Nitrate (NO3-) is a necessary nutrient for plants, whereas high-level of NO3- enters the food chain via water sources can be toxic and hazardous to the ecosystem [13]. The main culprits of NO3- pollution include agricultural runoff, fertilizer abuse, septic systems, industrial plants, and irrigation systems [4]. Currently, NO3- contamination has been a serious environmental concern as it can affect the quality of groundwater and surface water, causing health problems to humans [5, 6]. In an oxygen-deficient environment, such as in the digestive tract, NO3- can be reduced to more toxic nitrite [7, 8]. Nitrite can oxidize low ferritin into methemoglobin in the human body, causing the loss of oxygen transportation. It can also react with secondary amine compounds to produce carcinogenic nitrosamines [9]. Therefore, it is urgent to develop a novel strategy to remove NO3- pollution with high efficiency and selectivity.

Ammonia (NH3) is an important industrial ingredient widely used in fertilizers, pharmaceutical industries, and other areas [1012]. More importantly, it is also an essential energy source. The decomposition of ammonia is a low-cost and facile process; therefore, using ammonia as a hydrogen carrier is able to solve the hydrogen storage problem to a certain extent [13, 14]. With the increase of the global population, the demand for NH3 keeps increasing. Currently, NH3 is mainly produced via the Haber process, in which gaseous nitrogen (N2) and water gas are converted to NH3 under high temperature and high pressure with the assist of catalysts [15]. Every year, the synthesis of NH3 consumes about 2% of global energy, leading to serious carbon dioxide emission [1618]. Thus, it is urgent to develop a green synthesis of NH3 under ambient conditions.

In recent years, photocatalytic synthesis of NH3 has become a hot research frontier, where researchers are mainly focusing on the reduction of gaseous nitrogen [1922]. However, the research is still limited in the laboratory, because the strong bond energy of N ≡ N bonds and the poor solubility of N2 make the catalytic NH3 yield efficiency low [16]. Compared with N2, the required energy for the cleavage of N=O is only 21.68% as that of N ≡ N bonds. In addition, the solubility of NO3- is 40000 times higher than that of N2 [23]. Therefore, if the waste NO3- can be converted to NH3, the conversion of environmental hazards to valuable energy resources will be more sustainable for the planet.

Graphitic carbon nitride (g-C3N4) is a metal-free photocatalyst, which is a rising star in photocatalysis because it has the advantages of facile synthesis, wide sources, and good photocatalytic activity for redox reactions [2426]. In recent years, several kinds of g-C3N4-based photocatalysts have been prepared and applied in NO3- wastewater treatment, such as TiO2/g-C3N4 [27], Mn2O3/g-C3N4 [28], and g-C3N4@AgPd heterojunction [29]. However, the main aim is to convert NO3- to N2 to reduce environmental hazards. Compared with the generation of N2, the reduction of NO3- to NH3 is more challenging in terms of kinetics and thermodynamics, as it is an eight-electron process with multiple steps [30]. On the other hand, many ruthenium-containing compounds exhibit good catalytic properties because of their unique electronic structure [3134]. Meanwhile, ruthenium (Ru) can absorb the light in the visible spectrum, and it is being actively researched for solar energy technologies [35, 36]. Nevertheless, toxic reducing agents like ethylene glycol are usually required to reduce Ru3+ to Ru0, making the synthesis complex, dangerous, and unecofriendly [37]. Until now, Ru modified g-C3N4 photocatalysts for highly efficient NO3- reduction to NH3 have never been reported, and the reaction mechanism is not clear.

In this work, we report a green synthesis of Ru modified g-C3N4 nanosheets with significantly enhanced photocatalytic activity on the reduction of NO3- to NH3. Herein, waste green tea bags were used to reduce the Ru3+, and the photocatalytic activity of the optimized sample was 2.93-fold as that of bulk g-C3N4 under simulated sunlight irradiation. The material preparation process was ecofriendly without using strong reducing reagent. Based on experimental and theoretical studies, the introduction of Ru to g-C3N4 can not only boost the light absorption, the adsorption of NO3-, but also accelerate the separation of electron-hole pairs. The thermodynamic energy barrier for the rate determining step in NO3- reduction to the NH3 process is calculated to be less than 0.75 eV, which is much lower than the competing H2 generation (0.98 eV) and N2 formation (1.36 eV), leading to the preference of generating NH3. This work provides a novel approach to synthesize metallic particle-based photocatalysts for highly efficient photocatalytic NO3- reduction to synthesize NH3.

2. Experimental Section

2.1. Chemicals and Synthesis of Materials

All the used chemicals including ruthenium trichloride (RuCl3), sodium nitrate (NaNO3), sodium sulfate (Na2SO4), dicyandiamide, sulfuric acid (H2SO4), formic acid, and dimethyl sulfoxide (DMSO) are analytical reagent and used without further purification. These chemicals were purchased from Merck Australia.

2.2. Synthesis of g-C3N4 Nanosheets

g-C3N4 was prepared via the thermal polymerization of dicyandiamide. Firstly, 5 g of dicyandiamide was put in a 50 mL corundum crucible without the lid. Then, the crucible was transferred into the oven and heated from 25 to 540°C with the heating rate of 4°C/min. After heating at 540°C for 4 h in air, the bulk g-C3N4 was obtained. The g-C3N4 nanosheets were synthesized via the ultrasonic peeling of bulk g-C3N4 in water. In details, 500 mg of bulk g-C3N4 was added in 800 mL water and put under ultrasonic bath for 24 h. Finally, the ultrathin g-C3N4 nanosheets were successfully obtained and named as 2D-CN.

2.3. Synthesis of Ru/g-C3N4

Herein, we used waste green tea as the reduction regent to prepare Ru modified g-C3N4 (Scheme 1). To get simulated waste tea bags, 5 tea bags were put in boiled water for 20 min. Then, the 5 used tea bags were put in 500 mL and boiled for 30 min. After that, the tea water was gone through a filter for further use. A certain amount of RuCl3 and 0.5 g of g-C3N4 nanosheets was put into 150 mL prepared tea water and under magnetic stir for 24 h. Finally, the powder was washed by water and ethanol for 5 times and then dried at 60°C for 4 h. The amounts of RuCl3 used were 0.005, 0.01 and 0.038 g, and the obtained samples were named as CN-Ru-0.5, CN-Ru-1, and CN-Ru-3, respectively.

2.4. Characterizations

The X-ray diffraction (XRD) patterns of the samples were tested with a Bruker D8 Discover XRD with intense Cu radiation (40 kV and 40 mA, ) at room temperature. The morphology observation on the materials was carried out using a Zeiss Supra 55VP scanning electron microscope (SEM), with an operating voltage of 5-15 kV. The images of SEM-energy-dispersive X-ray spectroscopy (EDS) for elemental mapping were obtained with the Oxford Ultim Max. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDS mapping were performed on a double-corrected FEI Themis-Z 60-300 transmission electron microscopy (TEM) equipped with ChemiSTEM EDS detector system for ultra-high count rates. The X-ray photoelectron spectra (XPS) of the samples were tested with a Thermo Fisher Scientific K-Alpha+X-ray photoelectron spectroscopy. The UV-vis diffuse reflectance spectra (DRS) of prepared samples were obtained from a Perkin Elmber Lambda 950 UV/VIS/NIR spectrometer, using high-purity barium sulfate (BaSO4) as the blank reference. Photoluminescence (PL) spectra were obtained from a Shimadzu RF-6000 fluorescence spectrometer excited at 325 nm. The photocurrent, electrochemical impedance spectroscopy (EIS) and Mott-Schottky curves of the samples were obtained from an electrochemical working station (CHI-760E) in 0.1 M Na2SO4 solution (details shown in supplementary materials).

2.5. Photocatalytic Activity Testing

The photocatalytic nitrate reduction performance was carried out with a self-made quartz reactor under simulated sunlight irradiation. During the reaction process, the light intensity was 600 mW/cm2 (HSX-F300), and it was measured by a radiometer. The distance between the light and the reactor was 10 cm. Firstly, 20 mg of the samples was weighed and put into the reactor. Then, 95 mL of NaNO3 (10 mg/L) solution and 5 mL of formic acid (200 μL/L) were added. Before the reaction, argon was purged into the solution to remove air. After 2 h strong stir to make the powder catalysts uniformly dispersed in the solution, the light was turned on and the light irradiation lasted 4 h. In each hour, 3 mL of liquid was sampled from the reactor. The concentration of ammonium was measured by UV-vis spectrometer using Nessler’s reagent method and was further verified by 1H-Nuclear magnetic resonance spectroscopy (NMR) analysis. The concentration of NO3- and NO2- was measured by a high-pressure integrated capillary anion chromatography (IC). The exact details of Nessler’s reagent method, NMR, and IC are shown in the supplementary materials (Figure S1-S4).

2.6. Density Functional Calculation

The calculations of geometry structures and energies were carried out by density functional theory (DFT) with Vienna ab initio simulation package (VASP) [3841]. The exact details are presented in supplementary materials.

3. Results and Discussion

3.1. Structure and Morphology

In the XRD patterns of g-C3N4 (Figure S5 in supporting information, SI), two distinct peaks at 13.1 and 27.1° can be assigned to the (100) and (002) crystal plane diffraction (JCPDS 87-1526) [42]. After peeled off by ultrasonic powder, the peak intensity decreased, indicating the reduced crystallinity. Since the loaded amount of Ru is not high, it cannot be detected by XRD. Then, HRTEM and SEM were used to explore the morphology of the prepared samples. As shown in Figure 1(a), the g-C3N4 nanosheets showed an ultrathin film layer structure. The CN-Ru-1 sample had the same morphology as 2D-CN with Ru particles loaded on the g-C3N4 nanosheets (dark spots in Figure 1(b)). The size of the Ru particles was less than 5 nm. To further investigate the lattice, the Ru particles were zoomed in and displayed in Figure 1(c). The lattice distance of 0.234 nm can be indexed to the (100) of metallic Ru. Additionally, the high-angle annular dark-field (HAADF) image and energy dispersive spectroscopy (EDS) for Ru element mapping were obtained to further confirm the component of the CN-Ru sample. As exhibited in Figures 1(d) and 1(e) and Figure S6, Ru particles are uniformly distributed on g-C3N4 nanosheets. SEM image was also employed to study the morphology of CN-Ru-1 (Figure 1(f)). In the SEM image, we can only see the nanosheet structure of g-C3N4, since the size of Ru particles was too small to be observed by SEM.

Then, XPS was used to study the components of the prepared samples. Figures 2(a) and 2(b) are the narrow C 1 s spectra of 2D-CN and CN-Ru-1. The three peaks of 2D-CN at 293.6, 287.9, and 284.6 eV can be assigned to the conjugated π electrons and sp2-hybridized C and C–C bonds [10]. Compared with 2D-CN, two new peaks appeared in the narrow XPS spectra of C of CN-Ru-1. The peak at 286.1 eV was attributed to C–O which came from the green tea (Figure 2(b)) [43]. The other new peak at 280.2 eV is associated with Ru0 [44]. Both 2D-CN and CN-Ru-1 have 3 peaks in the narrow N 1 s spectra. In Figure 2(c), the three peaks at 404.4, 400.0, and 398.4 eV of 2D-CN are attributed to the charging effect, tertiary nitrogen, and sp2-hybridized nitrogen. Notably, the three peaks of N of CN-Ru-1 shifted to 404.9, 400.1, and 398.6 eV, which was caused by the interaction between 2D-CN and Ru [45].

Based on the XRD, SEM, TEM, and XPS results, it can be concluded that metallic Ru particle modified g-C3N4 nanosheets were successfully fabricated.

3.2. Enhanced Photocatalytic NO3- Reduction to NH3

Herein, we used photocatalytic NO3- reduction to NH3 to evaluate the catalytic activity of samples. After 4-hour simulate sunlight irradiation, all the samples exhibited apparent photocatalytic activity on the reduction of NO3- to NH3 (Figure 3(a)). The NH3 yield rate of 2D-CN was 1.126 mg/h/gcat, and it was 1.26-fold as that of bulk g-C3N4. When metallic Ru was used to modify the 2D-CN, the photocatalytic activity increased remarkably. Among all the samples, CN-Ru-1 was the most active one (2.627 mg/h/gcat), and its activity is 2.93 times higher than bulk g-C3N4. Cycle stability is crucial for the study and application of photocatalysts. After the fourth cycle, the NH3 yield of CN-Ru-1 was still 2.32 mg/h/gcat, which was 88.16% as the fresh catalyst, indicating good cycle stability for reusage. The use of formic acid did not influence of the detection by the accuracy of Nessler’s reagent method (Figure S7).

To confirm the source of NH3 generated, we carried on the 15N isotope experiment with Na15NO3. A series of 15NH4Cl solutions with different concentration were firstly used to generate a standard line, and then the concentration of photocatalytic reactions can be analyzed. As shown in Figure 3(c), the standard 1H-NMR spectrum of 15NH4+ had two peaks with a coupling constant of 73.14 Hz. In the solution after photocatalytic reaction of CN-Ru-1 with 15NO3-, two characteristic 15NH3 peaks were also found, and the calculated concentration of NH3 was 2.05 mg/L, which well matched the results of UV-vis spectrophotometry. The NMR results can confirm that the formation of NH3 is attributed to the photocatalytic reduction of NO3-.

Besides NH3, there might be some other products of the NO3- reduction. Herein, we used ion chromatography to measure the concentration of NO3- and NO2-. After 4 h light irradiation, bulk CN reduced 48.85% of the NO3- (10 mg/L), 50.77% of which was converted to NH3. Under the same condition, CN-Ru-1 eliminated 92.85% of NO3-, and the NH3 selectivity was 77.9%. The UV-vis spectrophotometry, 1H-NMR, and ion chromatography results confirmed that both the NO3- conversion rate and NH3 selectivity were significantly increased by CN-Ru-1, compared with bulk CN.

In Table S1, we summarized several typical g-C3N4, TiO2, and other materials for photocatalytic NO3- reduction in literature for comparison. Many materials can achieve highly efficient NO3- removal, whereas the ammonia selectivity is quite low. Pd/TiO2 can get a high NH3 selectivity of 76.9%, but its catalytic activity is really poor [46]. Nevertheless, the CN-Ru-1 reported in this work can simultaneously realize both high NO3- removal and high NH3 selectivity.

3.3. Mechanism of the Enhanced Performance

Light absorption is an essential step for photocatalytic reaction. Figure 4(a) shows the UV-vis DRS of bulk CN, 2D-CN, and CN-Ru samples. The absorption edge of bulk CN was at about 460 nm, indicating a bandgap of 2.70 eV. When bulk CN was converted to 2D-CN, the absorption edge had a blue shift to 430 nm, with a wider bandgap of 2.88 eV. When the 2D-CN was modified by metallic Ru particles, the absorption edge showed a redshift. Meanwhile, the light absorption in the visible light region was also significantly enhanced. The increased light absorption enabled to make the catalysts absorb more photons, boosting the solar energy utilization ratio. The density of states of g-C3N4 and CN-Ru is calculated by DFT (Figure 4(b)). Compared with g-C3N4, the introduction of metallic Ru can bring some metallicity to the composite catalysts, which is beneficial to the transfer and separation of photocatalytic generated electron-hole pairs [47].

In order to explore more direct evidence about the transfer and separation of photogenerated electron-hole pairs, the photocurrent response, EIS, and PL spectra of bulk CN, 2D-CN, and CN-Ru samples were tested. Figure 4(c) is the photocurrent response image. All the tested samples had apparent and rapid photocurrent response under light irradiation. Among all the samples, the photocurrent intensity of CN-Ru-1 was the strongest, which followed the same trend as the photocatalytic activity, indicating that CN-Ru-1 had the fastest separation and transfer of electrons and holes. The photocurrent slightly decreased in 200 s, which was caused by the decreased interaction between the powder catalysts and the ITO glass [48]. It does not mean the photocatalysts have poor stability. In the EIS Nyquist plot, the bulk CN had the largest arc radius because of its poor conductivity. The 2D-CN got a smaller arc radius as it had better conductivity than bulk CN. When the 2D-CN got modified by metallic Ru, the arc radius of the EIS Nyquist plot significantly decreased, because Ru had strong conductivity. The more metallic Ru loaded, the better conductivity the sample got. The PL spectra of bulk CN, 2D-CN, and CN-Ru samples were displayed in Figures 4(e) and 4(f). Bulk CN showed a broad and strong emission spectrum with profiles slightly deviating from a perfect Guanine curve centered at about 460 nm. Compared with bulk CN, the PL intensity of the other samples decreased significantly, which mean the recombination of photoinduced charge carriers was obstructed, and it is helpful for the enhanced photocatalytic activity [49].

Experimental studies on NO3- reduction have shown that Ru modified g-C3N4 plays an essential role in improving NH3 production compared with pure g-C3N4. To further understand the reaction mechanism and the origin of high activity and selectivity of CN-Ru, the density functional theory calculation is performed. NO3- adsorption, the first step of NO3- reduction, was first calculated, and the optimized adsorption structures, as well as the adsorption energies, are listed in Figures 5(a) and 5(b). The adsorption energy of NO3- on g-C3N4 was -1.85 eV, and it was -3.75 eV on CN-Ru. It is clearly seen that Ru modification increased the NO3- stability by 2.03 times compared with pristine g-C3N4, indicating the following NO3- reduction step proceeds more easily, consistent with the experimental observations.

Furthermore, metallic Ru particles involve a high spin density on Ru cluster and positive Bader charge, as well as a zero gap, which can enhance the electron transfer ability (Figures 5(c) and 5(d)). Since Ru cluster got a positive Bader charge, the photocatalytic generated electrons for g-C3N4 can be accumulated on Ru, making it as the active sites. At the same time, the CN-Ru system got some magnetic properties, and the spin density is concentrated on Ru. All these electronic structure results are responsible for the improved photocatalytic activity of CN-Ru.

To further understand the role of metallic Ru particles in the NO3- reduction process, we calculated the free energy diagrams of NO3- reduction on g-C3N4 and CN-Ru. Compared with the generation of NH3, there are two main competing reactional products for the photocatalytic reduction of NO3- including H2 and N2. For g-C3N4, it is very easy to take part in hydrogen generation reaction, while the formation of N2 is very difficult because a higher thermodynamic energy barrier of 2.33 eV should be overcome (Figure 6(a)). As for CN-Ru, the thermodynamic energy barriers of the formation of H2 and N2 are 0.98 and 1.36 eV (Figure 6(b)). All free energy curves are downhill from NO3- to HNO2, and although the reaction pathway is different, the thermodynamic energy barrier for the rate determining step in this process is all calculated to be less than 0.75 eV (Figure 6(c)), which is much lower than those of (0.98 eV) and (1.36 eV), suggesting that induced Ru cluster played a key role in the enhancement of the photocatalytic activity and selectivity for NO3- reduction to form NH3, in good agreement with the experimental observations.

Based on the experimental and calculated results, the mechanism for the enhanced photocatalytic NO3- reduction to NH3 activity can be summarized as follows (Scheme 2). Firstly, the introduction of metallic Ru caused a redshift of the absorption edge as well as broad absorption in visible light range, which boosted the absorption of photons and the utilization of solar light. Secondly, CN-Ru samples have much better conductivity and higher separation efficiency of photogenerated electron-hole pairs than g-C3N4. Thirdly, the high spin density of Ru cluster and its positive Bader charge can accumulate electrons, making the adsorption and cleavage of NO3- easier. More importantly, the thermodynamic energy barrier for the rate determining step in this process is all calculated to be less than 0.75 eV (Figure 6(c)), which is much lower than the competing H2 generation (0.98 eV) and N2 formation (1.36 eV), leading to the preference of generating NH3 and higher activity.

Using green tea as reducing reagent to prepare nanomaterials is promising because it is cost-effective. More importantly, the operation and storage are much safer than using strong reducing reagent like sodium borohydride. The tea polyphenols of green tea are regarded as active ingredients to achieve reduction reactions [50, 51]. Meanwhile, the average reduction potential of green tea is about 0.219 V, indicating that the approach reported in this work can be expanded to the synthesis of other metal-based materials [52]. In Figure S8, we summarized the elements that can be reduced to zero valent theoretically, including Tc, Ru, and Cu, which would potentially use the proposed approach [53].

4. Conclusion

In summary, we used a facile and green approach to synthesize novel metallic Ru modified g-C3N4 nanosheets as photocatalysts for enhanced NO3- reduction to NH3. The optimal sample (CN-Ru-1) had the highest NH3 yield rate of 2.627 mg/h/gcat, and it was 2.93 times as that of bulk CN. After 4-hour light irradiation, CN-Ru-1 eliminated 92.85% of NO3-, and the NH3 selectivity was 77.9%. After four cycles, the sample still had good photocatalytic performance (88.16% as the fresh catalyst). NMR and 15N isotope labeling provided solid evidence that the N of NH3 was from the reduction of NO3-. The g-C3N4 nanosheets modified by metallic Ru particles have stronger light absorption, better conductivity, and more rapid separation of electron-hole pairs. With the enhanced adsorption energy of NO3- and the low thermodynamic energy barriers, the photocatalytic activity and selectivity were increased significantly. The results and findings of this work may provide a new platform for the facile and green synthesis of metal particle modified photocatalysts for reducing NO3- to NH3 under ambient conditions.

Data Availability

The authors declare that the main data supporting the findings in this study are available within the article and its supplementary information. Additional data are available from the corresponding authors upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

Authors’ Contributions

D. H., J.R., H.K.S., X.B., and B.J.N. proposed the project and designed the experiment. D.H., J.R., and W.W. prepared the materials and carried out the characterizations. H.A. and M.G. contributed to the TEM characterizations. Y.W. carried out the DFT calculations. The manuscript was written and edited by all the authors. Derek Hao and Jiawei Ren contributed equally to this work.

Acknowledgments

This work is supported by an Australian Research Council (ARC) Future Fellowship (FT160100195), the National Key Research and Development Program of China (2016YFA0602900), the National Natural Science Foundation of China (21673220), Department of Science and Technology of Sichuan Province (2017GZ0051), and Jilin Province Science and Technology Development Program (20190201270JC, 20180101030JC). Part of the computational time is supported by the High Performance Computing Center of Jilin University and Jilin Province, as well as Network and Computing Center of Changchun Institute of Applied Chemistry. Derek Hao acknowledges the technical support of Beijing NBET Technology Co., Ltd. The authors also acknowledge the Sydney Analytical (USYD), CTWW, MAU, and Chem Lab of UTS for the lab supports.

Supplementary Materials

S1: electrochemical characterizations. S2: 1H-NMR and HPIC. S3: density functional calculation. (Supplementary Materials)

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